When it is attempted to improve productivity by increasing a total fineness of a carbon fiber bundle for the purpose of reducing production cost of a carbon fiber, there are many problems in terms of practical use and production technology and the cost was not reduced sufficiently.
In order to solve these problems, Patent Document 1 has proposed the technology in which a scorch upon a flame proof treatment is reduced using a carbon-fiber-precursor fiber bundle having a high roundness and further a large single-fiber fineness and a carbon fiber bundle which contains few interlaced single fibers, has excellent spreadability and productivity despite of large total fineness is obtained.
Patent Document 2 has proposed a polymer which does not require the flame-proof treatment. Further, Patent Documents 3, 4 and 9 have proposed the technology to enhance oxygen permeability of the carbon-fiber precursor fiber to control an oxygen concentration in a flame-proof fiber evenly and enhance tensile strength and tensile elastic modulus of obtained carbon fiber by using a monomer having a bulky side chain as a copolymerizable component of a copolymer.
Further, Patent Document 5 has proposed the technology to reduce thermal storage inside the fiber bundle by progressing the flame-proof while heated air is penetrated inside the fiber bundle on a mesh-shaped roller for PAN-based carbon-fiber-precursor fiber bundles.
Patent Document 6 has proposed the technology in which by measuring an isothermal exothermic curve of the carbon-fiber-precursor fiber bundle using a heat flow type differential scanning calorimeter, a content of a carboxylic group-containing vinyl monomer is optimized, a cross-section double structure after the flame-proof treatment is reduced even when burning at high speed is performed, and the productivity and the elastic modulus of the carbon fiber bundle can be balanced. Patent Document 7 has proposed the technology to produce the carbon fiber bundle with high performance by copolymerizing acrylamide to obtain a highly hydrophilic polyacrylonitrile copolymer.
Stabilization of the fiber in each step is also very important for reducing the production cost of the carbon fiber. For example, gelation of a spinning neat solution in a spinning step sometimes leads to a step trouble, and it is required to enhance thermal stability of the spinning neat solution. In Patent Document 8, the thermal stability when the spinning neat solution is kept at high temperature of about 80° C. is exponentially enhanced by esterifying methacrylic acid that is an accelerating component of the flame-proof reaction of the polymer.
A technique using a prepreg obtained by impregnating a fiber for reinforcement with a matrix resin composed mainly of a thermosetting resin is available as one of methods of molding fiber-reinforced composite materials, and such a composite material is used for a wide range of uses from sport/leisure-related uses to uses for aircrafts. The fiber-reinforced composite material using an intermediate base material composed of the prepreg is formed by laminating the prepreg followed by heating or heating/pressurizing to cure the thermosetting resin that is the matrix resin.
The technique using the prepreg is more excellent in fiber strength utilization than VARTM method. When the molding in large size is formed, generally it is desirable that the matrix resin be high flow. The matrix resin with low flow causes an occurrence of voids. However, when the matrix resin is high flow, micro ondulation of the fiber occurs and mechanical physical property of the molding in large size is reduced. The mechanical physical property in the molding in large size largely depends on its thickness, and when the thickness of the molding is increased, compression strength is reduced. Patent Documents 10 and 11 have proposed to prevent the reduction of various physical properties by making the matrix resin low flow.
When the fiber-reinforced fabric is used as a fiber base material, a resin film in which a resin was applied onto a film is attached to the fiber-reinforced fabric to make a prepreg, which is laminated in several layers, and the layers are heated and pressurized in an autoclave formation. In that case, the entire fabric is impregnated sufficiently with the resin and a good molding is obtained. The impregnation with the resin is also very good regardless of structure and fiber cross-sectional shape of the fiber-reinforced fabric. However, in RTM molding and vacuum bag formation, the resin is injected into the fiber base material, and thus, the resin having good fluidity and so-called a low viscosity resin is generally used as the resin. Thus, comparing with the conventional autoclave formation, the cost of forming the fiber base material having a larger weight per unit area is described to be advantageous, but the impregnation with the resin is largely influenced by the viscosity of the resin, the weight per unit area of the fiber fabric, inter-fiber voids, a single-fiber diameter, and the like, which was problematic.
Patent Document 12 has proposed the carbon fiber bundle having a ratio of a major axis and a minor axis (major axis/minor axis) of 1.05 to 1.6 in a fiber cross-section of a single-fiber in the carbon fiber bundles composed of a single-fiber of multiple carbon fibers as the carbon fiber bundle that simultaneously satisfies bundle integrity, the impregnation with the resin and cloth quality of obtained cloth and has the high strength.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2008-202207    Patent Document 2: Japanese Unexamined Patent Application, Publication No. H01-132832    Patent Document 3: Japanese Unexamined Patent Application, Publication No. H02-84505    Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2006-257580    Patent Document 5: Japanese Unexamined Patent Application, Publication No. H02-6625    Patent Document 6: Japanese Unexamined Patent Application, Publication No. 2000-119341    Patent Document 7: Japanese Unexamined Patent Application, Publication No. H04-281008    Patent Document 8: Japanese Unexamined Patent Application, Publication No. 2007-204880    Patent Document 9: Japanese Unexamined Patent Application, Publication No. H02-84505    Patent Document 10: Japanese Unexamined Patent Application, Publication No. H01-161040    Patent Document 11: Japanese Unexamined Patent Application, Publication No. H02-169658    Patent Document 12: Japanese Unexamined Patent Application, Publication No. 2002-242027